Lead zirconate titanate containing lead is a typical piezoelectric material, and is used in a variety of piezoelectric devices such as an actuator, an oscillator, a sensor, and a filter. However, it has been pointed out that, when a piezoelectric device containing lead is once scrapped and exposed to acid rain, the lead content in the piezoelectric material may be transferred into the soil to adversely affect the ecosystem. Accordingly, in order to exclude lead from piezoelectric devices, research and development on lead-free piezoelectric materials are actively conducted.
Currently, a typical lead-free piezoelectric material that has been widely researched is a piezoelectric material containing potassium niobate (KNbO3). However, when a piezoelectric material containing potassium is synthesized, it has been difficult to weigh raw material (for example, potassium carbonate) powder precisely at an intended molar ratio owing to hygroscopic property of the raw material powder. Further, the piezoelectric material containing potassium niobate (KNbO3) has deliquescence, and hence piezoelectricity of piezoelectric ceramics containing potassium niobate is sometimes degraded with the passage of time.
NPL 1 reports, as a piezoelectric material free of lead and potassium, a solid solution (hereinafter referred to as “NN—BT”) of sodium niobate (NaNbO3) and barium titanate (BaTiO3), which is an antiferroelectric. Non Patent Literature 1 discloses that a piezoelectric constant d33 of piezoelectric ceramics containing sodium niobate and barium titanate at a ratio of 9:1 is 147 pC/N.
PTL 1 provides a method of manufacturing a niobic acid-based piezoelectric porcelain having a high Curie temperature and satisfactory piezoelectric property. Patent Literature 1 discloses that a niobic acid-based piezoelectric porcelain that is a solid solution of NN—BT and strontium titanate (SrTiO3) has a piezoelectric constant d33 of 14 to 126 μm/V.